In the transmission front end of optical communication, continuous light is modulated using an input data signal, so that an electric signal is converted into an optical signal to be transmitted. In high-speed long-distance optical transmission, an external modulation method is commonly used so as to avoid a problem with chirping. A method is disclosed of improving the quality of an optical signal generated by an optical modulator using digital signal processing in an optical transmitter employing an external modulation method (see, for example, Japanese Laid-open Patent Publication No. 2012-129606).
In a multi-level modulation method in which optical signals at a plurality of amplitude levels are generated on a transmission side, optical modulation is performed by linearly amplifying a converted analog electric signal. As illustrated in FIG. 1, the relationship between the output of a DAC and the output characteristic of an optical modulator has a linear characteristic in a certain range. The output of the DAC is connected to the input of a driving circuit. Since a driving signal amplified by the driving circuit is input into the optical modulator, the linearity illustrated in FIG. 1 corresponds to the input-output characteristic of the entire analog part from a DAC output to an optical output amplitude. For the sake of convenience, a region where an optical output is below the linearity is called a “saturated nonlinear range” and a region where an optical output is above the linearity is called a “super linear range”. In order to output an optical signal with less distortion, the optical transmitter is used in a range where linear input-output characteristic is obtained. The linearity of the optical output characteristic with respect to the DAC output depends on the performance of the driving circuit and the optical modulator and also depends on a gain value set for the driving circuit. Before operation, the gain of the driving circuit is usually optimized to check the range where the input-output characteristic of the part from the output of a DAC to the output amplitude of an optical modulator has linearity.
The output of the DAC and the gain of the driving circuit decrease with time. The input-output characteristic may fluctuate depending on the changes in temperature and power supply voltage. During an operation, the optical transmitter is used with a margin in the range where the linearity is obtained. With the decreases in the output of the DAC and the gain of the driving circuit, the output power of an optical signal decreases and the noise immunity of a main optical signal is deteriorated. A configuration is therefore desired with which the gain of the driving circuit and/or the output of the DAC can be readjusted even during an operation.
A configuration is disclosed with which a linear electro-optical conversion is realized and the difference between an I-channel signal level and a Q-channel signal level is suppressed (see, for example, Japanese Laid-open Patent Publication No. 2011-232553). In this configuration, a low-frequency signal component is superimposed on the driving amplitude of a modulator, an optical signal is monitored, and the amplitudes of an I-channel driving signal and a Q-channel driving signal are adjusted to optimum driving amplitudes.